1. Field of the Invention
This disclosure relates to a switching power source device of nonlinear control method.
2. Description of Related Art
Each of figures FIGS. 12 A to 12C illustrates a circuit block diagram and an operation waveform diagram illustrating a switching power source device adopting a nonlinear control method in accordance with conventional examples. FIG. 12A illustrates a hysteresis window method, FIG. 12B illustrates a fixed ON-time with bottom detection method, and FIG. 12C illustrates a fixed OFF-time with upper detection method, respectively. In addition, each of the switching power source devices illustrated in FIGS. 12A to FIG. 12C is a step-down DC/DC converter to generate a desired output voltage OUT by stepping down an input voltage IN.
Compared to a switching power source device of a linear control method (e.g., a voltage mode control method or a current mode control method), the switching power source device of a nonlinear control method has a advantage that a high load transient response can be obtained with a simple circuit construction.
On the other hand, a switching power source device of a nonlinear control method drives a comparator by utilizing an output ripple voltage (i.e., the ripple component of the output voltage OUT), and then a switching control of an output transistor is performed. Therefore, based on the construction described above, an output ripple voltage with a large amplitude (i.e., peak value) to some extent is required to detect the output ripple voltage correctly. Therefore, using of a capacitor having a relatively large ESR [Equivalent Series Resistance] (e.g., a conductive polymer type) cannot be avoided, which limits selection of a part or increases cost.
Furthermore, a known technique to drive a comparator in a stable manner based on a forcible injection of a ripple component to the reference voltage Vref provided to a comparator (i.e., a ripple injection technique, in other words, a ripple is combined with other voltages) is proposed. If adopting this ripple injection technique, even in if an amplitude of the output ripple voltage is not so large, stable switching control can be performed, and a multilayer ceramic capacitor with small ESR can be used as an output capacitor.
In addition, as a conventional technique related to the aforementioned technique, Japanese patent publication No. 2010-35316 can be listed.
However, a DC value of the reference voltage Vref to which a ripple component is injected fluctuates according to a duty of a switch voltage Vsw (i.e., a pulse voltage generated at one end of the output transistor) used for a generation of the ripple component. Especially, as the amount of the ripple injection (i.e., an amplitude of the ripple component injected to the reference voltage Vref) is increased, the more the DC value of the reference voltage Vref to which the ripple component is injected fluctuates.
Therefore, in the conventional switching power source device, there is a problem that an output voltage accuracy or a load regulation characteristic (i.e., a stability of the output voltage OUT against a load fluctuation) can be worsened as a trade off, contrary to an increase of an amount of ripple injection for enhancing the stability or jitter characteristic of the switching operation.